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DiRienzo, Nicholas; Niemelä, Petri T.; Hedrick, Ann V. and Kortet, Raine (2016): Adult bacterial exposure increases behavioral variation and drives higher repeatability in field crickets. In: Behavioral Ecology and Sociobiology, Vol. 70, No. 11: pp. 1941-1947

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Among-individual differences in behavior are now a widely studied research focus within the field of behavioral ecology. Furthermore, elements of an animal's internal state, such as energy or fat reserves, and infection status can have large impacts on behaviors. Despite this, we still know little regarding how state may affect the expression of behavioral variation. Recent exposure to pathogens may have a particularly large impact on behavioral expression given that it likely activates costly immune pathways, potentially forcing organisms to make behavioral tradeoffs. In this study, we investigate how recent exposure to a common bacterial pathogen, Serratia marcescens, affects both the mean behavioral expression and the among-individual differences (i.e., variation) in boldness behavior in the field cricket, Gryllus integer. We find that recent pathogen exposure does not affect mean behavioral expression of the treatment groups, but instead affects behavioral variation and repeatability. Specifically, bacterial exposure drove large among-individual variation, resulting in high levels of repeatability in some aspects of boldness (willingness to emerge into a novel environment), but not others (latency to become active in novel environment), compared to non-infected crickets. Interestingly, sham injection resulted in a universal lack of among-individual differences. Our results highlight the sensitivity of among-individual variance and repeatability estimates to ecological and environmental factors that individuals face throughout their lives. Animals are known to express consistent among-individual differences in behavior, also known as animal personalities, such that some individuals are always more bold, aggressive, or active relative to others. Yet it is relatively unknown how factors such as energy reserves and exposure to pathogens impact these differences in behavior. Here we investigated how exposure to a pathogenic bacteria as adults affects both the mean behavioral expression and the among-individual differences. Our results show that pathogen exposure results in large levels of among-individual differences in some aspects of boldness behavior (willingness to expose oneself to risk), but not others (latency to become active in a novel environment). These results highlight the sensitivity of behavioral differences to elements of state, such as infection status.

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